Re: Prevent all uses of DFP when unsupported (PR c/91985)

[Jeff Law <law at redhat dot com>]

On 11/22/19 9:40 AM, Joseph Myers wrote:
> Code that directly uses _Decimal* types on architectures not
> supporting DFP is properly diagnosed ("error: decimal floating-point
> not supported for this target"), via a call to
> targetm.decimal_float_supported_p, if the _Decimal32, _Decimal64 or
> _Decimal128 keywords are used to access it.  Use via mode attributes
> is also diagnosed ("unable to emulate 'SD'"); so is use of the
> FLOAT_CONST_DECIMAL64 pragma.  However, it is possible to access those
> types via typeof applied to constants or built-in functions without
> such an error.  I expect that there are ways to get an ICE from this;
> certainly it uses a completely undefined ABI.
> 
> This patch arranges for the types not to exist in the compiler at all
> when DFP is not supported.  As is done with unsupported _FloatN /
> _FloatNx types, the global tree nodes are left as NULL_TREE, and the
> built-in function machinery is made to use error_mark_node for them in
> that case in builtin-types.def, so that the built-in functions are
> unavailable.  Code handling constants is adjusted to give an error,
> and other code that might not work with the global tree nodes being
> NULL_TREE is also updated.
> 
> Bootstrapped with no regressions for x86_64-pc-linux-gnu.  Also tested
> with no regressions for cross to aarch64-linux-gnu, as a configuration
> without DFP support.  OK to commit (the changes that aren't C front-end 
> changes)?
> 
> gcc:
> 2019-11-22  Joseph Myers  <joseph@codesourcery.com>
> 
> 	PR c/91985
> 	* builtin-types.def (BT_DFLOAT32, BT_DFLOAT64, BT_DFLOAT128)
> 	(BT_DFLOAT32_PTR, BT_DFLOAT64_PTR, BT_DFLOAT128_PTR): Define to
> 	error_mark_node if corresponding global tree node is NULL.
> 	* tree.c (build_common_tree_nodes): Do not initialize
> 	dfloat32_type_node, dfloat64_type_node or dfloat128_type_node if
> 	decimal floating-point not supported.
> 
> gcc/c:
> 2019-11-22  Joseph Myers  <joseph@codesourcery.com>
> 
> 	PR c/91985
> 	* c-decl.c (finish_declspecs): Use int instead of decimal
> 	floating-point types if decimal floating-point not supported.
> 
> gcc/c-family:
> 2019-11-22  Joseph Myers  <joseph@codesourcery.com>
> 
> 	PR c/91985
> 	* c-common.c (c_common_type_for_mode): Handle decimal
> 	floating-point types being NULL_TREE.
> 	* c-format.c (get_format_for_type_1): Handle specified types being
> 	NULL_TREE.
> 	* c-lex.c (interpret_float): Give an error for decimal
> 	floating-point constants when decimal floating-point not
> 	supported.
> 
> gcc/lto:
> 2019-11-22  Joseph Myers  <joseph@codesourcery.com>
> 
> 	PR c/91985
> 	* lto-lang.c (lto_type_for_mode): Handle decimal floating-point
> 	types being NULL_TREE.
> 
> gcc/testsuite:
> 2019-11-22  Joseph Myers  <joseph@codesourcery.com>
> 
> 	PR c/91985
> 	* gcc.dg/c2x-no-dfp-1.c, gcc.dg/gnu2x-builtins-no-dfp-1.c: New
> 	tests.
> 	* gcc.dg/fltconst-pedantic-dfp.c: Expect errors when decimal
> 	floating-point not supported.
OK
jeff